Is a prepaid legal services plan that is funded solely by a portion of fees collected by a union, not an employer subject to ERISA?

It is intended that those union members who choose to participate will contribute a flat fee per month, a portion of which will go towards funding this plan.

If I'm not mistaken, as long as participation is voluntary, there are no employer contributions and the sponsor is the union and not an employer, it is not subject to ERISA.

Please advise if I am correct.

Thanks.